This invention relates to an improved method for efficiently and precisely adhering and centering particles to the tacky areas on a surface containing an array of tacky and non-tacky areas and removing particles from the non-tacky areas without removing the particles from the tacky areas. This net adhering of particles on the surface only to the tacky areas is termed net population of the surface (with the particles).
The placement of particles, such as electrically conductive solder, on contact pads is critical to the adoption of array style semiconductor packages such as ball grid arrays (BGA). Such placement is also critical in the attachment of integrated circuits (IC) to packages or printed circuit boards through xe2x80x9cflip chipxe2x80x9d processes. Recent attempts have been made to improve, for example, solder ball interconnects, such that more reliable and/or less costly solder connections are made in electronic applications. Despite these efforts, there are still problems associated with the handling and transfer of particles, primarily conductive particles such as solder balls to form solder bumps, on the contact pads of electronic devices. However, there is still a need for further improvements, particularly with regard to the efficiency, precision, and robustness of the population process(es). As a result, improved products and methods remain a primary objective in this art area.
The present invention is directed to an improved method for mounting particles on a surface having an array of tacky and non-tacky areas thereon, comprising the steps of:
(a) obtaining the surfaces having an array of tacky and non-tacky areas thereon;
(b) flowing the particles across the surface to allow particles to contact the tacky areas and adhere thereto; and
(c) removing the excess particles not adhered to the tacky areas;
whereby heating to a temperature of at least 30xc2x0 C. takes place prior to step (c).
Further embodiments of the invention include a step of agitating the surface having the array of tacky and non-tacky areas with an agitation rate of less than 1000 cycles per minute, using ionized air to partially neutralize electrostatic charges, use of holding times to obtain improved results and transfer of particles such as in manufacture of an electronic device.
In a further embodiment heating is not essential provided the particles are applied to the surface with tacky and non-tacky areas by agitation, such as vibration and use of hold times of the particles on the surface prior removal of excess particles and transfer to a substrate such as in manufacture of an electronic device.